Fischer–Tropsch Synthesis for the Production of Sustainable Aviation Fuel: Formation of Tertiary Amines from Ammonia Contaminants

Fischer–Tropsch synthesis combined with product workup is a promising route toward synthetic aviation fuel from renewable hydrogen and carbon sources like biomass, CO2, and waste. Cost savings can be achieved by reducing the number of gas treatment steps in new plants, but the consequence of contaminants in the feed needs investigation. While feeding 2.6 ppmV ammonia to a Fischer–Tropsch reactor, it was shown that ammonia was predominantly chemically converted into organic amines, with most nitrogen found in the water phase (89%), followed by heavy wax (7%) and light wax (1%). The concentration difference between water and light wax was shown to be due to the post-condensation separation of amines on polarity. Amines up to a chain length of 120 were detected in the heavy wax with MALDI-FT-ICR-MS, which, in combination with the high nitrogen content, suggests that amines have a similar chain growth probability compared to the main hydrocarbon products. Detailed product analysis with three independent analytical techniques showed that tertiary N,N-dimethylalkylamines were by far the most abundant amine class. This suggests that ammonia is decomposed on the cobalt surface and, potentially as a dimethylamine fragment, incorporated in the growing chain. Further evidence was obtained from the abundance of trimethylamine and from the reconciled nitrogen product analysis up to C100, which showed that the amine product distribution followed from naphtha onward the same ASF kinetics as alkanes and oxygenates while being distinctively different from the alkene distribution. The presented findings provide further avenues for studies of the Fischer–Tropsch reaction mechanism and indicate the opportunity of cost saving on gas treatment, while further validation is required to assess the impact on hydrocracking and product quality.


SPME-GCMS
The water samples were subjected to headspace solid-phase microextraction (HS-SPME) at a temperature of 60°C, both on the liquid and in the headspace.In both cases, a multipurpose PMDS/DVB/CWR fiber was employed for extraction.The analytes obtained from the water sample were injected into the gas chromatography-mass spectrometry (GC-MS) system using a splitless injection technique.The GC system was equipped with an SPSil 5CB column measuring 50m x 0.32mm x 1.2µm.The initial temperature of the column was set at 40°C for 1 minute and then increased at a rate of 25°C per minute for a duration of 10 minutes until reaching 325°C.Helium was used as the carrier gas with a flow rate of 2.0 ml/min.The mass selective detector source temperature was maintained at 230°C.To extract structural information and facilitate identification, the fragmentation patterns of the eluting peaks were analyzed.This information was then utilized to identify and annotate relevant components in the chromatogram.Additionally, single ion monitoring was employed to enhance sensitivity and selectivity for specific compound classes of interest, such as primary, secondary, and tertiary amines and alcohols and acids.
Modelling amine distribution Starting from the total hydrocarbon product distribution two models were made to derive from them an amine distribution.The first model assumes I) the observed experimentally split between gas, light wax and heavy wax as the basis for the amine partitioning with water/light wax split governed by the HLB, II) Light amines are adjusted to fit the observations: C1 is 90% reduced, C2 is doubled and C3 is 8.7 times increased versus hydrocarbons, III) Amines have the same chain growth probability compared to hydrocarbons, and IV) preservation of nitrogen balance holds.The second model deviates from the third assumption of model 1 by assuming that amines have a slightly higher chain growth probability compared to hydrocarbons.

Figure
Figure S1 (a) Mass spectrum obtained for the water sample diluted 1:100 vol:vol with MQ/MeOH containing 0.1% FA measured with direct infusion ESI-TOFMS and (b) corresponding relative amine content distribution in the water sample.

Figure
Figure S3 Overlay of relative distribution profiles of alkylamines in the heavy wax sample as observed by DI-ESI-TOFMS after dissolution in toluene/methanol (50/50) containing 0.1% FA (black) and paraffins as observed by GC-FID (red).

Figure
Figure S4 Mass spectra obtained for the heavy wax sample mixed with DHB (A, 1:1; B, 1:5; and C, 1:10 mass:mass) during MALDI-FT-ICR-MS.The red line indicates the amine distribution.

Figure
Figure S6 Summation of single quadrupole mass spectra with Flow Injection Analysis using toluene/methanol on the light wax sample indicating a distinct CH 2 distribution (Δm/z = 14) differing from the alkylamine series by a m/z of 16.

Figure
Figure S8 MS/MS fragmentation spectra of C16 and C12 primary linear amines in (A, C) 1 ppm reference solution and (B, D) the water sample.The respective parent ions are indicated by yellow stars.The different fragmentation patterns with same parent ion mass are indicative of structural differences.

Figure
Figure S9 Overlay of EICs of m/z 188.222 for the water sample spiked with 1 ppm dodecylamine (Red) and spiked with 1 ppm N,Ndimethyldecylamine (Black).The N,N-dimethyldecylamine coelutes with the peak of the water sample (6.60 min), the dodecylamine clearly elutes at a later time (7.59 min) from the column, validating the dimethylated nature of the amine in the water sample.The insert shows the BPI chromatogram for both spiking experiments.

Table S1
Overview of concentrations in the water sample of ammonia as measured with Ion Chromatography (IC) and amines measured with Ion Chromatography, Single Quadrupole MS (SQMS) and ESI-TOFMS and consolidated results.All concentrations are expressed in ppmw N.

Table S2 :
Amine concentration in water, light wax and heavy wax as experimentally established and derived with model 1 and model 2 from the hydrocarbon product distribution.Values are expressed in ppmw N.

Table S3 :
Paraffin, Olefin, Oxygenate and Amine concentration (wt%) in the various outlet streams